The use of a photopolymerizable dry film as a resist for etching and plating a substrate to form a printed circuit is disclosed in U.S. Pat. No. 3,469,982. Although this use has gained widespread acceptance, it has the disadvantage of requiring "wet" development of the resist image, i.e., the area of photopolymerizable layer that is unexposed in the imagewise photoexposure step, is removed by washing away with solvent which does not dissolve the exposed area of the layer.
Numerous attempts have been made to make a photopolymerizable layer that is developable dry, i.e., the unexposed area of the layer is removed from the exposed area of the layer merely by peeling away of the support sheet, wherein the exposed area of the layer remains on the substrate to which it was laminated and the unexposed area of the layer remains adhered to the support sheet. None of these attempts have been commercially successful. No dry-developing photopolymerizable dry film resist is commercially available.
To detail some of these attempts, U.S. Pat. No. 3,770,438 discloses a dry-developing photopolymerizable layer on a polymer film support operating according to the principle of the polymerizable monomer being present in the layer in a quantity in excess of the absorptive capacity of the binder so that a thin layer of substantially free monomer is present on the surface of the photopolymerizable layer. This free monomer is detectible on the surface of a substrate such as copper as an oily residue when the layer is peeled off of the copper prior to any photoexposure of the layer. The free monomer polymerizes when photoexposed to actinic radiation, to adhere the exposed area of the layer to the copper more strongly than to the film support. The greater affinity of the monomer for the film support than the copper in the unexposed area of the layer enables the unexposed area of the layer to remain adhered to the film support. Different ways for achieving an excess of monomer are used in the patent, e.g., using a hydrocarbon polymer binder and a very large proportion of monomer (77.5% in Ex. 3), using a chlorocarbon polymer binder having a lesser absorptive capacity for the monomer, thereby reducing the monomer content to 40-50% (Ex. 9, 13, 15 & 16), or using a combination of a chlorocarbon polymer (M. wt. about 20,000) and higher molecular weight (about 60,000) poly(methylmethacrylate) to decrease the flow of the layer. The amount of chlorocarbon polymer used ranged from 24.6 to 41.4 by wt., the amount of hydrocarbon polymer ranged from 3 to 29.4% by wt. and the amount of monomer ranged from 24.2 to 65.6% by wt. (Ex. 1, 2, 5-8, 12, 14, and 16). The disadvantage of the dry-developing photopolymerizable layer of this patent was that either the flow of the layer at ordinary room temperature was too great or the fidelity of the image reproduced by the layer was less than desired for the manufacture of printed circuits. As the flow of the photopolymerizable layer was reduced in an attempt to provide a layer that could be supplied in the form of a roll, as required for commercial acceptance, the layer would either not fail cohesively at the boundary between the unexposed area and exposed area of the layer with a smooth, sharp break to give the image fidelity desired or would not fail cohesively at all, i.e., even the unexposed area of the layer would adhere to the substrate when the film support was peeled away. Layers that gave good image fidelity upon dry development were so tacky that it was difficult to remove a coversheet desired to be laminated to a surface of the layer. None of the Examples use a coversheet.
Japanese Patent Publication 35,722 published Sept. 28, 1978 discloses earlier Japanese patent publications 43-22901 and 47-7728; these correspond approximately to U.S. Pat. Nos. 3,353,955 and 3,770,438 (mentioned above), respectively. According to Publication 35,722, these earlier publications disclose a dry-developing photopolymerizable layer composed of polymer binder, unsaturated monomer, and photopolymerization initiator, with the properties of the polymer binder, such as molecular weight, softening point, crystallinity, miscibility with other materials, and adhesion towards the substrate as being important properties. Pub. 35,722 also discloses that Japanese Patent Publication 38-9663 discloses polyvinyl butyral, polyvinylacetate, polyvinylpyrrolidone, gelatin, coumarone indene resin, silicone resin, rubber, etc. and that Pub. 47-7728 discloses binder of vinylidene chloride, cellulose ether, synthetic rubber, polyvinyl acetate copolymer, polyacrylate, polyvinylchloride, etc. Pub. 35,722 discloses that tests made of photopolymerizable compositions with these binders did not always obtain satisfactory results. Pub. 35,722 purports to solve the problem by using a chlorinated polyolefin, disclosing chlorinated polyethylene and chlorinated polypropylene as being preferred, as the polymer binder, in the dry-developing photopolymerizable layer as a plating resist in which the resist is removed after plating and the substrate under the resist is etched.
U.S. Pat. No. 4,058,443 discloses the same background art as referred to in Pub. 35,722. U.S. Pat. No. 4,058,443 discloses further that the mechanical strength of photopolymerizable layers used for dry development are apt to have insufficient mechanical strength which is important when the layer is used as a tenting resist. U.S. Pat. No. 4,058,443 purports to solve this problem by having the binder of the photopolymerizable layer, in addition to chlorinated polyolefin, consist of 10 to 90% of a straight-chained polymeric material having sufficient compatibility with the other components of the layer and having a molecular weight of at least 10,000, with a range up to 2,000,000 being disclosed. The chlorinated polyolefin is also claimed to have a polymerization degree of 600 to 20,000 which for chlorinated polypropylene (64% by wt. Cl) corresponds to a molecular weight of about 34,000 to 2,200,000. The resist utility disclosed in this patent is as an etching resist. The layers of this patent are either too flowable or when not too flowable, they have deficient image fidelity upon dry development. In addition, the conditions for peeling away the support film to achieve dry development are too exacting for commercial practice. As such these layers have nothing more than laboratory application.
The need has also arisen for a particular type of dry film photoresist, namely one which is a mask for molten solder and which can remain as a permanent coating on the printed circuit. Such dry film solder mask would offer greater image resolution and fidelity than silk-screen methods now predominantly used. In addition to the usual requirements of dry film photoresist used for plating or etching, the solder mask utility has the additional requirements that the dry film has to encapsulate the exposed surfaces of conductors, withstand the molten solder without embrittlement or loss of adhesion to the printed circuit and without chemical degradation, and be flame retardant, moisture resistant, and a dielectric as well.
Belgian Pat. No. 860,181 granted Apr. 27, 1978, discloses photopolymerizable solder mask compositions provided as a layer sandwiched between a film support and a coversheet. The layer is laminated to a printed circuit by application of heat and pressure and is imagewise exposed and then developed in the same manner as in U.S. Pat. No. 3,469,982, namely by "wet" development to obtain the solder mask (resist image) on the printed circuit. Layer thickness of from 0.0008 cm (0.003 inch) to 0.025 cm (0.01 inch) is disclosed, but all the Examples use a layer thickness of 0.01 cm (0.004 inch) because that was what was required to encapsulate the conductors of the printed circuitry. Conductors were often at least 0.005 cm in height requiring thicker solder mask layers in order to be able to encapsulate the conductors without excessive thinning out of the layer or puncture of the layer by the conductors.
U.S. Pat. No. 4,127,436 discloses a special vacuum lamination process for applying the solder mask to the printed circuit, enabling the thickness of the photopolymerizable layer to be reduced. Example 1 discloses layer thickness of 0.002 inch covering conductors of 0.002 to 0.0044 inch in height. Encapsulation of the conductor by the solder mask is shown in FIG. 3. The solder mask used in this process were wet developed but the concept of dry development is disclosed.
Photopolymerizable solder masks have been commercially available for several years first in the 0.01 cm thickness and eventually in the 0.0075 cm thickness all for development by wet development.
A dry-developing dry film resist having commercial utility would be desirable. To summarize the art discussed hereinbefore, the wet-developing dry film photoresist has achieved commercial utility and so has the wet-developing solder mask. Dry-developing dry film photoresists have been proposed and worked on but have not achieved commercial utility. Dry-developing solder masks have been proposed but without any hint of how to achieve a commercially useful one. The need for a commercially useful dry-developing resist film remains unfulfilled.